Critical-state wedges, ice glaciers, and salt sheets have many geometric and mechanical similarities. Each has a tapering geometry and moves along a basal detachment. Their motions result from the combined effects of internal deformation and basal sliding. Wedge deformation and geometry, basal conditions, and overpressure (pore fluid pressure less hydrostatic pore fluid pressure) development within the substrate interact with each other in this mechanically coupled system. However, the nature of this interaction is poorly understood. In order to investigate this coupled system, we have developed two-dimensional poromechanical finite-element models with porous fluid flow in sediments. We have simulated the advance of a salt-sheet wedge across poroelastic sediments in this study. We emphasize that our results have applications beyond salt wedges to both critical-state wedges and ice glaciers. Overpressure develops within the substrate over time during the advance of the wedge. The magnitude of the overpressure influences the wedge geometry and the wedge advance rate. Lower overpressure results in a thicker and steeper wedge geometry, and a slower advance rate, while higher overpressure favors a thinner, wider and more flattened wedge geometry, and a faster advance rate. This study provides key insights into the links between wedge geometry, basal shear stress and overpressure in substrates.

We model the evolution of a salt diapir during sedimentation and study how deposition and salt movement affect stresses close to the diapir. We model the salt as a solid visco-plastic material and the sediments as a poro-elastoplastic material, using a generalized Modified Cam Clay model. The salt flows because ongoing sedimentation increases the average density within the overburden sediments, pressurizing the salt. Stresses rotate near a salt diapir, such that the maximum principal stress is perpendicular to the contact with the salt. The minimum principal stress is in the circumferential direction, and drops near the salt. The mean stress increases near the upper parts of the diapir, leading to a porosity that is lower than predicted for uniaxial burial at the same depth. We built this axisymmetric model within the large-strain finite-element program Elfen. Our results highlight the fact that forward modeling can provide a detailed understanding of the stress history of mudrocks close to salt diapirs; such an understanding is critical for predicting stress, porosity, and pore pressure in salt systems.

We compare an evolutionary with a static approach for modeling stress and deformation around a salt diapir; we show that the two approaches predict different stress histories and very different strains within adjacent wall rocks. Near the base of a rising salt diapir, significantly higher shear stresses develop when the evolutionary analysis is used. In addition, the static approach is not able to capture the decrease in the hoop stress caused by the circumferential diapir expansion, nor the increase in the horizontal stress caused by the rise of the diapir. Hence, only the evolutionary approach is able to predict a sudden decrease in the fracture gradient and identify areas of borehole instability near salt. Furthermore, the evolutionary model predicts strains an order of magnitude higher than the strains within the static model. More importantly, the evolutionary model shows significant shearing in the horizontal plane as a result of radial shortening accompanied by an almost-equivalent hoop extension. The evolutionary analysis is performed with ELFEN, and the static analysis with ABAQUS. We model the sediments using a poro-elastoplastic model. Overall, our results highlight the ability of forward evolutionary modeling to capture the stress history of mudrocks close to salt diapirs, which is essential for estimating the present strength and anisotropic characteristics of these sediments.

Channel-levee systems are responsible for constructing deep sea fans, among the largest sedimentary deposits on Earth. Levee height plays a key role in defining the volume and texture of the material that is deposited in the bounding levees, and thus the morphology of submarine fans. Models of channel formation and evolution generally assume that the levees aggrade in response to the cumulative overspill of turbidity flows, and that their height is controlled by these flows. In contrast, we show that levee growth in the Ursa Basin (Gulf of Mexico) is limited by the mechanical strength of the levee, not the flow behavior. While many studies document sidewall failures in levee systems, our poro-mechanical model is the first to demonstrate that collapse of levees is a large-scale, deep-seated process driven by the interaction of levee formation and high fluid pressure. Rapid deposition of a regional sand unit induced large fluid overpressure in the underlying mud, which preconditioned the system for levee failure, which then fed a large volume of sediment back into the channel-levee system. Long-lived levee failures continually reintroduced previously deposited levee material back into the channel system. This implies that a large volume of sediment is continuously recycled, which has not been previously understood. Turbidite flow models generally assume that flows progressively lose their fine-grained component due to levee overspill as they traverse the channel. In contrast, we show a mechanism by which fine-grained material can re-enter the system in large quantities, and this has significant and broad importance for models of channel and fan evolution. We also show that that levee failure introduces significant unconformities, in contrast with the common assumption that levees offer complete and high-resolution records of climate, tectonics, and sea level.

We present a one-dimensional model that couples the thermodynamics of hydrate solidification with multiphase flow to illuminate how gas vents pierce the hydrate stability zone in the world's oceans. During the propagation phase, a free-gas/hydrate reaction front propagates toward the seafloor, elevating salinity and temperature to three-phase (gas, liquid, and hydrate) equilibrium. After the reaction front breaches the seafloor, the temperature gradient in the gas chimney dissipates to background values, and salinity increases to maintain three-phase equilibrium. Ultimately, a steady state is reached in which hydrate formation occurs just below the seabed at a rate necessary to replace salt loss. We show that at the Ursa vent in the Gulf of Mexico, the observed salinity and temperature gradients can be simulated as a steady state system with an upward flow of water equal to 9.5 yr -1 and a gas flux no less than 1.3 mÃ¢ yr -1. Many of the world's gas vents may record this steady state behavior, which is characterized by elevated temperatures and high salinities near the seafloor.

We introduce dual-mode dilative failure with flume experiments. Dual-mode dilative failure combines slow and steady release of sediments by breaching with periodic sliding, which rapidly releases an internally coherent wedge of sediments. It occurs in dilative sandy deposits. This periodic slope failure results from cyclic evolution of the excess pore pressure in the deposit. Sliding generates large, transient, negative excess pore pressure that strengthens the deposit and allows breaching to occur. During breaching, negative excess pore pressure dissipates, the deposit weakens, and ultimately sliding occurs once again. We show that the sliding frequency is proportional to the coefficient of consolidation. We find that thicker deposits are more susceptible to dual-mode dilative failure. Discovery of dual-mode dilative failure provides a new mechanism to consider when interpreting the sedimentary deposits linked to submarine slope failures.

Breaching is a type of retrogressive submarine slope failure associated with pore pressure drops in both space and time, and this drop strengthens the failing deposit. Breaching is characterized by a near-vertical failure surface that retreats with a relatively constant velocity, on the order of a millimeter per second. Breaching is controlled by interactions between shear-dilation-generated pore pressure drops and pore pressure dissipation through intergranular fluid flow. Laboratory measurements show that shear dilation in a deposit increases with increasing effective stress ratio between the major principal effective stress and the minor principal effective stress as well as decreasing confining stress. We present a two-dimensional numerical model that indicates how effective stress ratio and confining stress produce spatially varying dilation, affecting the mechanics of breaching. Experimental results show that dilation in a breaching deposit increases with proximity to the failure surface. As a result, the maximum magnitude of pore pressure drop is very close to the failure surface. The numerical model confirms that the sediment release is dominated by pore pressure dissipation through intergranular fluid flow in the horizontal direction. This allows the erosion rate to be treated as a constant in the vertical direction. Numerical model results also show that because dilation decreases with increasing vertical depth, the deposit becomes less stable with depth, suggesting a potential upper limit for the thickness of the deposit undergoing breaching.

In Resedimented Boston Blue Clay (RBBC), a low-plasticity glacio-marine illitic mudrock, the ratio of the horizontal to vertical permeability (the permeability anisotropy, rk) increases from 1.2 to 1.9 as the porosity decreases from 0.5 to 0.37 and the permeability decreases by more than 1 order of magnitude. Backscattered Scanning Electron Microscope (BSEM) images taken at formation stress levels reveal that particles rotate perpendicular to the axial loading direction by 22Â°, with larger particles rotating more significantly and achieving more uniform alignment than smaller particles. We show experimentally that preferred platy particle orientation can explain our permeability anisotropy measurements. The permeability anisotropy of mechanically compressed mudrocks is minimal, <2.5. We use a novel approach (cubic specimens) to measure the evolution of permeability anisotropy in different directions on the same specimen, unlike most other methods. Modified analytic techniques allow calculation of the permeability anisotropy for a specimen using directional constant head permeability methods. A better understanding of the evolution of permeability anisotropy during sediment burial is important for modeling subsurface transport processes, including hydrocarbon migration and contaminant transport, as well as estimating in situ conditions such as pore pressure, overpressure, and effective stress.

Mud-rich mass transport deposits (MTDs) have a microfabric that is significantly different from bounding non-deformed mudstones at similar depths in the first 200 m of burial. Core samples from the Integrated Ocean Drilling Program Expedition 308, Ursa Basin, Gulf of Mexico sample many well identified MTDs. These MTD mudstones have higher clay mineral fabric intensities than compositional equivalent mudstones either at a given porosity or a given depth. Clay mineral fabric intensity was quantified using high resolution X-ray texture goniometry and confirmed by visual inspection on backscattered electron micrographs imaged on argon-ion milled surfaces. Enhanced clay-mineral fabric intensities in MTD mudstones are interpreted to result from remolding and shearing after mass movement, where the initially deposited clay mineral flocs have been mechanically disaggregated and physio-chemical forces of attraction overcome. Recognition of enhanced microfabrics has important implications for seismic anisotropy as well as for shallow fluid flow.

Sandstone pressures follow the hydrostatic gradient in Miocene strata of the Mad Dog field, deep-water Gulf of Mexico, whereas pore pressures in the adjacent mudstones track a trend from well to well that can be approximated by the total vertical stress gradient. The sandstone pressures within these strata are everywhere less than the bounding mudstone pore pressures, and the difference between them is proportional to the total vertical stress. The mudstone pressure is predicted from its porosity with an exponential porosity-versus-vertical effective stress relationship, where porosity is interpreted from wireline velocity. Sonic velocities in mudstones bounding the regional sandstones fall within a narrow range throughout the field from which we interpret their vertical effective stresses can be approximated as constant. We show how to predict sandstone and mudstone pore pressure in any offset well at Mad Dog given knowledge of the local total vertical stress. At Mad Dog, the approach is complicated by the extraordinary lateral changes in total vertical stress that are caused by changing bathymetry and the presence or absence of salt. A similar approach can be used in other subsalt fields. We suggest that pore pressures within mudstones can be systematically different from those of the nearby sandstones, and that this difference can be predicted. Well programs must ensure that the borehole pressure is not too low, which results in borehole closure in the mudstone intervals, and not too high, which can result in lost circulation to the reservoir intervals.

In situ stress and pore pressure are key parameters governing rock deformation, yet direct measurements of these quantities are rare. During Integrated Ocean Drilling Program (IODP) Expedition #319, we drilled through a forearc basin at the Nankai subduction zone and into the underlying accretionary prism. We used the Modular Formation Dynamics Tester tool (MDT) for the first time in IODP to measure in situ minimum stress, pore pressure, and permeability at 11 depths between 729.9 and 1533.9âmbsf. Leak-off testing at 708.6âmbsf conducted as part of drilling operations provided a second measurement of minimum stress. The MDT campaign included nine single-probe (SP) tests to measure permeability and in situ pore pressure and two dual-packer (DP) tests to measure minimum principal stress. Permeabilities defined from the SP tests range from 6.53âÃâ10â17 to 4.23âÃâ10â14âm2. Pore fluid pressures are near hydrostatic throughout the section despite rapid sedimentation. This is consistent with the measured hydraulic diffusivity of the sediments and suggests that the forearc basin should not trap overpressures within the upper plate of the subduction zone. Minimum principal stresses are consistently lower than the vertical stress. We estimate the maximum horizontal stress from wellbore failures at the leak-off test and shallow MDT DP test depths. The results indicate a normal or strike-slip stress regime, consistent with the observation of abundant active normal faults in the seaward-most part of the basin, and a general decrease in fault activity in the vicinity of Site C0009.

Modeling studies suggest that fluid permeability is an important control on the maintenance and distribution of pore fluid pressures at subduction zones generated through tectonic loading. Yet, to date, few data are available to constrain permeability of these materials, at appropriate scales. During IODP Expedition 319, downhole measurements of permeability within the uppermost accretionary wedge offshore SW Japan were made using a dual-packer device to isolate 1 m sections of borehole at a depth of 1500 m below sea floor. Analyses of pressure transients using numerical models suggest a range of in-situ fluid permeabilities (5E-15 to 9E-17 m2). These values are significantly higher than those measured on core samples (2E-19 m2). Borehole imagery and cores suggests the presence of multiple open fractures at this depth of measurement. These observations suggest that open permeable natural fractures at modest fracture densities could be important contributors to overall prism permeability structure at these scales.

How the micro-scale fabric of clay-rich mudstone evolves during consolidation in early burial is critical to how they are interpreted in the deeper portions of sedimentary basins. Core samples from the Integrated Ocean Drilling Program Expedition 308, Ursa Basin, Gulf of Mexico, covering seafloor to 600 meters below sea floor (mbsf) are ideal for studying the micro-scale fabric of mudstones. Mudstones of consistent composition and grain size decrease in porosity from 80% at the seafloor to 37% at 600 mbsf .Argon-ion milling produces flat surfaces to image this pore evolution over a vertical effective stress range of 0.25 (71 mbsf) to 4.05 MPa (597 mbsf). With increasing burial, pores become elongated, mean pore size decreases, and there is preferential loss of the largest pores. There is a small increase in clay mineral preferred orientation as recorded by high resolution X-ray goniometry with burial.

As part of the government response to the Deepwater Horizon blowout, a Well Integrity Team evaluated the geologic hazards of shutting in the Macondo Well at the seafloor and determined the conditions under which it could safely be undertaken. Of particular concern was the possibility that, under the anticipated high shut-in pressures, oil could leak out of the well casing below the seafloor. Such a leak could lead to new geologic pathways for hydrocarbon release to the Gulf of Mexico. Evaluating this hazard required analyses of 2D and 3D seismic surveys, seafloor bathymetry, sediment properties, geophysical well logs, and drilling data to assess the geological, hydrological, and geomechanical conditions around the Macondo Well. After the well was successfully capped and shut in on July 15, 2010, a variety of monitoring activities were used to assess subsurface well integrity. These activities included acquisition of wellhead pressure data, marine multichannel seismic profiles, seafloor and water-column sonar surveys, and wellhead visual/acoustic monitoring. These data showed that the Macondo Well was not leaking after shut in, and therefore, it could remain safely shut until reservoir pressures were suppressed (killed) with heavy drilling mud and the well was sealed with cement.

We compare four approaches to geomechanical modeling of stresses adjacent to salt bodies. These approaches are distinguished by their use of elastic or elastoplastic constitutive laws for sediments surrounding the salt, as well as their treatment of fluid pressures in modeling. We simulate total stress in an elastic medium and then subtract an assumed pore pressure after calculations are complete; simulate effective stress in an elastic medium and use assumed pore pressure during calculations; simulate total stress in an elastoplastic medium, either ignoring pore pressure or approximating its effects by decreasing the internal friction angle; and simulate effective stress in an elastoplastic medium and use assumed pore pressure during calculations. To evaluate these approaches, we compare stresses generated by viscoelastic stress relaxation of a salt sphere. In all cases, relaxation causes the salt sphere to shorten vertically and expand laterally, producing extensional strains above and below the sphere and shortening against the sphere flanks. Deviatoric stresses are highest when sediments are assumed to be elastic, whereas plastic yielding in elastoplastic models places an upper limit on deviatoric stresses that the rocks can support, so stress perturbations are smaller. These comparisons provide insights into stresses around salt bodies and give geoscientists a basis for evaluating and comparing stress predictions.

We use a fully coupled poroelastoplastic geomechanical model to study how stresses and pore pressures evolve in sediments bounding a spherical salt body. Drained analyses (pore pressures remain hydrostatic) demonstrate that sediments yield in response to loading by the salt, which leads to a redistribution of stresses and to deformations larger than predicted by poroelastic or solid Coulomb-plastic models. Undrained analyses (overpressures develop while no dissipation occurs) illustrate that salt loading induces pore pressures that extend kilometers away from the salt body. We also model the flow and consequent dissipation that occur in the sediments because of this undrained salt loading. We show that with time, the pressure field dissipates and expands. The dissipation process takes millions of years, which suggests that pore-pressure perturbations caused by salt loading should still be present in mudstones near many salt bodies. Under drained conditions, stress perturbations generate low minimum principal stresses above and below the salt, resulting in convergence of pore pressure and minimum principal stress at these locations. Such conditions are challenging to drill through. In undrained systems, sharp drops in pore pressure may occur above and below the salt, whereas both the pore pressure and the minimum principal stress rise next to the salt. In contrast to previous models that do not couple changes in stress to changes in pore pressure, the coupled approach presented here has the potential to predict in-situ stresses and pore pressures more accurately in a wide variety of geologic settings.

n the Ursa Basin, Gulf of Mexico, in situ mudstone permeability near the seafloor declines from 1.1x10**(-16)x10**(-19) to 5.8 m2 over a depth of 578 m. We can reproduce this in situ permeability-porosity behavior through consolidation experiments in the laboratory. We use uniaxial constant-rate-of-strain consolidation experiments to measure permeability-porosity relationships and derive in situ permeabilities of 31 mudstone samples collected at Integrated Ocean Drilling Program (IODP) Sites U1324 and U1322. Although these mudstones have similar grain-size distributions, permeability at a given porosity varies significantly between the samples due to small variations in composition or fabric. We calculate an upscaled permeability relationship based on the observed permeability variation in the samples and determine a resultant large-scale permeability anisotropy of around 30. Based on this upscaled relationship and observations of in situ pressure, we calculate upward fluid flow rates of 0.5 mm/yr. We find that given the observed compressibility, permeability, and the geologic forcing at Ursa, overpressures are predicted as observed in the subsurface. The primary mechanism for overpressure generation at Ursa is sediment loading due to rapid burial. Low vertical permeabilities, accompanied by high sedimentation rates, can cause severe overpressure near the seafloor, which controls fluid flow and can reduce slope stability as observed in the Mississippi Canyon region. Such flow systems, especially at intermediate depths on passive margins, are important due to their control over macroscale behavior such as topographic gradient of continental slopes and submarine landslides, but have been largely understudied in the past.

Breaching is a style of retrogressive subaqueous slope failure controlled by dilation and consequent pore pressure drop; it has the potential to generate turbidity currents that build thick successions of turbidites. We present pore pressure measurements made during breaching failure, as well as a physical model that shows how the pore pressure field within the failing deposit is connected to the erosion rate associated with the failure surface. We show that breaching can occur in any dilative granular material. Conditions for breaching could be common on the continental shelf, making it an important mechanism in transferring sediment into the deep ocean. A dynamic equilibrium exists between the slope failure and the pore pressure dissipation during breaching. This equilibrium leads to a way to estimate the rate of sediment release from breaching using a simple material property, the coefficient of consolidation. Contrary to previous work, we find that the erosion rate is independent of the dilation of the deposit due to the coupling between erosion and pore pressure dissipation. The equilibrium between the erosion and pore pressure dissipation decouples the steady-state pore pressure field from the permeability of the deposit; this is the first time this behavior has been recognized in sediment failures.

Apron 1 in the Shallow Auger Fan System records the transition from ponded deposition in the lobe complex to bypass at the top of the channel complex. The lobe complex, at the base of Apron 1, exhibits characteristics typical of ponded apron deposits: it onlaps the basin margin, exhibits a concentric isopach pattern, has a lobe geometry in amplitude extraction, and is composed of continuous seismic

Development of a preferred orientation of clay minerals is investigated in response to changes in vertical effective stress and composition in resedimented material (Boston Blue Clay). Boston Blue Clay (BBC), an illitic glacio-marine clay composed of 57% clay-sized particles (<2 ÃÆÃ Â½ÃâÃÂ¼m) was resedimented and step-loaded to a vertical effective stress of 0.1 MPa, 1 MPa, 6 MPa, and 10 MPa. These four samples show a small increase in preferred orientation of mica and chlorite with increasing vertical effective stress. Furthermore, pure BBC powder was admixed with silica (silt-sized quartz) in five different ratios of BBC:silica to form sediments with 57%, 52%, 48%, 44%, and 36% < 2 ÃÆÃ Â½ÃâÃÂ¼m particles and were loaded to a maximum vertical effective stress of 2.4 MPa. Mica preferred orientation decreases significantly with decreasing clay content at one stress state. We used and compared three different techniques to characterize and quantify the preferred orientation of phyllosilicates: transmission-mode X-ray texture goniometry, reflection-mode X-ray texture goniometry, and grain and pore networks imaged using secondary and backscattered electron micrographs gathered on argon-ion-milled surfaces. Preferred orientation of the clay minerals shows good agreement between transmission-mode X-ray texture goniometry and reflection-mode X-ray texture goniometry. Mercury porosimetry further illuminates vertical effective stress and compositional controls on microfabric.

We study the three-phase (Liquid + Gas + Hydrate) stability of the methane hydrate system in marine sediments by considering the capillary effects on both hydrate and free gas phases. The capillary pressure, a measure of the pressure difference across a curved phase interface, exerts a key control on the methane solubility in Liquid + Hydrate (L + H) and Liquid + Gas (L + G) systems. By calculating the L + H and L + G solubilities as a function of water depth (pressure) and pore size (interface curvature), we show how the solubility requirements for forming both gas hydrate and free gas can be met in a three-phase zone. The top of the three-phase zone shifts upward in sediments as the water depth increases and the mean pore size decreases. The thickness of the three-phase zone increases as the distribution of pore sizes widens. The top of the three-phase zone can overlie or underlie the bulk three-phase equilibrium depth. At Blake Ridge, we predict that the three-phase zone is 27.7 m thick and that the top of the three-phase zone lies 13 m above the predicted bulk equilibrium depth. This reconciles the observation of the bottom-simulating reflector (BSR) at Blake Ridge that is shallower than the predicted bulk equilibrium depth. In contrast, at Hydrate Ridge where water depth is shallower, we predict that the three-phase zone is 20.4 m thick and that the top of the three-phase zone lies 0.7 m below the predicted bulk equilibrium depth. Our model, which predicts an upward shift in the top of free gas occurrence with increasing water depth (pressure), is compatible with worldwide observations that the BSR is systematically shifted upward relative to the bulk equilibrium depth as water depth (pressure) is increased.

In mudstones of the Ursa Basin, Gulf of Mexico, the volume of voids to solids, or void ratio, ranges from 4 (porosity = 80%) at the seafloor to 0.6 (porosity = 37%) at 600 m below seafloor. This seven-fold change in void ratio can only be described by a compaction model that includes greater sediment stiffening with stress than has commonly been used in geotechnical or geological approaches. Through uniaxial consolidation experiments, we show that specific volume (v = 1 + void ratio) declines as an exponential function of effective stress. We use this relationship to successfully predict in-situ overpressures at Integrated Ocean Drilling Program (IODP) Sites U1322 and U1324. This technique can be used around the world to describe sediment compaction and predict pore pressure in the first 1000 m below seafloor. Rapid sediment consolidation near the seafloor provides the fluid source that generates overpressure despite the fact that these sediments have high permeability. Ultimately sediment consolidation is a first order control on when and at what depth overpressure will be generated in the subsurface. This in turn will impact whether submarine landslides are expected and the regional gradient of continental margins.

Resistivity images from Integrated Ocean Drilling Program (IODP) Site U1322 on the Mississippi fan (Gulf of Mexico) show borehole failure as (1) low-resistivity bands interpreted as breakouts and (2) high-resistivity bands. Both features occur as opposing pairs on opposite sides of the borehole, and have similar azimuthal orientations and widths. Failures occur at depths of 90-216 m in sediments very rich in expansive (smectite-illite) clays of 40%-50% porosity that are younger than 65 ka. The low-resistivity breakouts resemble similar features in other IODP boreholes from southwest Japan and offshore Oregon. The high-resistivity features are unknown in other boreholes. Estimates of stress magnitudes based on the overburden stress and the extensional tectonic environment in the Gulf of Mexico predict that the borehole was at failure. Experiments were conducted on cores with lithologies equivalent to those of the borehole failure localities from IODP Site U1322 and adjacent Site U1324. These experiments suggest an elastic-plastic deformation with strains of 10%-15% before reaching a plastic yielding. In the experiments, strain softening during plastic deformation ranges from 0% to 20%. Physically the experimental samples show a combination of lateral bulging and discrete conjugate shears. These experiments suggest that the resistive areas in the borehole are an initial state of bulging, or extrusion, into the borehole. We call these extrusive failures "breakins" to distinguish them from traditional breakouts. Extrusion into borehole decreases the amount of conductive borehole fluid between the bulging sediment and the resistivity tool, increasing the resistivity signal. The high residual strength of the sediment prevents disaggregation and spalling. Where spalling has developed, breakouts occur. This analysis is the first documentation of this incipient stage of borehole failure.

At a given porosity, mudstone permeability increases by an order of magnitude for clay contents ranging from 57% to 36% (<2 ÃâÃÂµm). This increase in vertical permeability results from a dual-porosity system that develops through three mechanisms: (1) silt bridging preserves large pore throats, (2) stress bridges inhibit clay particle alignment, and (3) local clay particle compression within stress bridges alters pore throat size distribution. Uniaxial consolidation experiments on resedimented clay-silt mixtures illuminate how permeability varies as a function of clay fraction during burial. Backscattered electron microscope images show that silty mixtures have larger pore throats and fewer aligned clay particles than do more clay-rich mixtures. We describe the permeability of clay-silt mixtures with a geometric mean model. Our method provides a promising framework for modeling of mudstone permeability as a function of clay fraction and porosity. How permeability and consolidation evolve during burial affects the ability of mudstones to seal CO2 and hydrocarbons in the subsurface, how mudstones behave as gas reservoirs, and under what conditions mudstones will be overpressured. Dual-porosity systems have fundamentally different transient flow and solute transport behaviors.

Subsurface sediment remobilization and fluid flow processes and their products are increasingly being recognized as significant dynamic components of sedimentary basins. The geological structures formed by these processes have traditionally been grouped into mud volcano systems, fluid flow pipes and sandstone intrusion complexes. But the boundaries between these groups are not always distinct because there can be similarities in their geometries and the causal geological processes. For instance, the process model for both mud and sand remobilization and injection involves a source of fluid that can be separate from the source of sediment, and diapirism is now largely discarded as a deformation mechanism for both lithologies. Both mud and sand form dykes and sills in the subsurface and extrusive edifices when intersecting the sediment surface, although the relative proportions of intrusive and extrusive components are very different, with mud volcano systems being largely extrusive and sand injectite systems being mainly intrusive. Focused fluid flow pipes may transfer fluids over hundreds of metres of vertical section for millions of years and may develop into mud volcano feeder systems under conditions of sufficiently voluminous and rapid fluid ascent associated with deeper focus points and overpressured aquifers. Both mud and sand remobilization is facilitated by overpressure and generally will be activated by an external trigger such as an earthquake, although some mud volcano systems may be driven by the re-charge dynamics of their fluid source. Future research should aim to provide spatio-temporal Ã¢â¬ËinjectiteÃ¢â¬â¢ stratigraphies to help constrain sediment remobilization processes in their basinal context and identify and study outcrop analogues of mud volcano feeders and pipes, which are virtually unknown at present. Further data-driven research would be significantly boosted by numerical and analogue process modelling to constrain the mechanics of deep subsurface sediment remobilization as these processes can not be readily observed, unlike many conventional sediment transport phenomena.

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.

Pore fluid overpressures in four reservoir sandstones in the Auger Basin, deepwater Gulf of Mexico, are similar across the basin, suggesting that these sandstones are hydraulically connected over distances of 420 km. Small overpressure gradients within them suggest upward flow rates between 1 and 20mm/year. At the crest of these sandstones, pore pressure equals or exceeds the least principal stress, and we interpret that high fluid pressure is fracturing the caprock and driving flow vertically. A well drilled into the crest of the Auger sandstones confirmed the presence of extreme overpressures that converge on both the least principal stress and the overburden stress. Above these zones,

The Ursa Basin, at ~1,000 m depth on the Gulf of Mexico continental slope, contains numerous Mass Transport Deposits (MTDs) of Pleistocene to Holocene age. IODP Expedition 308 drilled three sites through several of these MTDs and encompassing sediments. Logs, sedimentological and geotechnical data were collected at these sites and are used in this study for input to basin numerical models. The objective of this investigation was to understand how sedimentation history, margin architecture and sediment properties couple to control pore pressure build-up and slope instability at Ursa. Measurements of porosity and stress state indicate that fluid overpressure is similar at the different sites (in the range of 0.5-0.7) despite elevated differences in sedimentation rates. Modeling results indicate that this results from pore pressure being transferred from regions of higher to lower overburden along an underlying more permeable unit: the Blue Unit. Overpressure started to develop at ~53 ka, which induced a significant decrease in FoS from 45ka, especially where overburden is lower.

During sea-level drop, water and gas pressures within oceanic hydrate systems can exceed the total vertical stress and this can drive slope failure and gas venting. We investigate this behavior with a multi-phase fluid and heat flow model. During sea-level drop, fluid pressures drop much less than the total stress due to both the high gas compressibility and hydrate dissociation. In permeable sediments, hydrate dissociation, water expulsion and gas mobility combine to induce underpressure and downward water flow from the seafloor. This study provides a causal mechanism for slope failure and fluid exchange that occur in hydrate systems during sea-level fall.

Borehole failures are a conspicuous feature of the logging-while-drilling resistivity images at Integrated Ocean Drilling Program Sites U1322 and U1324. Failures appear as irregular zones of low resistivity on opposite sides of the well bore (resembling traditional breakouts) and also as zones of high resistivity flanked by narrower low-resistivity intervals. The failures show a consistent eastâwest trend at both sites and with depth in each. The inferred SHmin directions are 85Â°â265Â° and 91Â°â271Â°, respectively, at Sites U1322 and U1324. SHmax at Sites U1322 and U1324 is oriented subparallel to the overall southerly slope of the Gulf of Mexico slope in this region. At Site U1322, SHmin is perpendicular to en echelon extensional fractures along the margin of a submarine landslide.

We completed 151 radiographs and 12 X-ray computed tomography (CT) scans from whole cores taken during Integrated Ocean Drilling Program (IODP) Expedition 308. In Brazos-Trinity Basin IV, 29 radiographs and 1 X-ray CT scan were taken at Sites U1319 and U1320. In Ursa Basin, 122 radiographs and 11 X-ray CT scans were taken at Sites U1322 and U1324. These radiographs and X-ray CT scans were completed to select undisturbed portions of the core for experiments and to qualitatively assess the presence of inclusions and variation in the whole-core soil samples from the expedition. The imaging was performed at three locations: Pennsylvania State University, Massachusetts Institute of Technology, and Rice University.

Pore water overpressures (u*) within mudstones beneath Brazos-Trinity Basin IV (deepwater Gulf of Mexico, offshore Texas) are greater than 70% of the hydrostatic vertical effective stress (σ&#8242; vh ) [λ* = 0.7 = (u*/σ&#8242; vh )]. These results are compatible with recent observations that suggest sedimentation rates in this region are rapid (6 mm/a). We compare the petrophysical properties and pore pressures within a 127-m-thick package of mudstone penetrated at two locations: Integrated Ocean Drilling Program (IODP) sites U1319 and U1320. Site U1319 is at the margin of Brazos-Trinity Basin IV, whereas Site U1320 lies at its center, beneath 180 m of turbidite fill. Experimentally derived preconsolidation stresses and an in situ pore pressure measurement record overpressure at Site U1319 and Site U1320 (λ* &#8764; 0.2 to 0.8 and λ* &#8764; 0.8, respectively). We use these data to define an average vertical effective stress gradient. Assuming that void ratio (e) is proportional to the log of vertical effective stress (σ&#8242; v ), we predict pore pressures (u) throughout the mudstone at both sites using bulk density data. Overpressures are greater at Site U1320 due to rapid deposition of the overlying turbidites. However, a large fraction of the overpressure induced by the turbidite load applied at Site U1320 has dissipated by drainage into the overlying basin fill. High overpressures near the seafloor drive shallow fluid flow, reduce slope stability, and may explain large submarine landslides.

Overpressures measured with pore pressure penetrometers during Integrated Ocean Drilling Program (IODP) Expedition 308 reach 70% and 60% of the hydrostatic effective stress () in the first 200 meters below sea floor (mbsf) at Sites U1322 and U1324, respectively, in the deepwater Gulf of Mexico, offshore Louisiana. High overpressures are present within low permeability mudstones where there have been multiple, very large, submarine landslides during the Pleistocene. Beneath 200 mbsf at Site U1324, pore pressures drop significantly: there are no submarine landslides in this mixture of mudstone, siltstone, and sandstone. The penetrometer measurements did not reach the in situ pressure at the end of the deployment. We used a soil model to determine that an extrapolation approach based on the inverse of square route of time () requires much less decay time to achieve a desirable accuracy than an inverse time (1/t) extrapolation. Expedition 308 examined how rapid and asymmetric sedimentation above a permeable aquifer drives lateral fluid flow, extreme pore pressures, and submarine landslides. We interpret that the high overpressures observed are driven by rapid sedimentation of low permeability material from the ancestral Mississippi River. Reduced overpressure at depth at Site U1324 suggests lateral flow (drainage) whereas high overpressure at Site U1322 requires inflow from below: lateral flow in the underlying permeable aquifer provides one mechanism for these observations. High overpressure near the seafloor reduces slope stability and provides a mechanism for the large submarine landslides and low regional gradient (2ÃÂ°) offshore from the Mississippi delta.

The publisher regrets that when the above article was printed, there were a series of errors in the text. The full correct article is printed on the following pages. We apologize for any inconvenience this may have caused. The Publisher. Available online 3 August 2008.

Organic carbon and total nitrogen stable isotopes are reported for sediments drilled during Integrated Ocean Drilling Program Expedition 308. Brazos-Trinity Basin IV sediments exhibited a broad range in organic carbon 13C ranging between â27 and â20 (13C average = â24.1) compared to Ursa Basin sediments that are generally more depleted in 13C (13C average = â25.7). Bulk 15N values across all basins ranged from â2.7 to 8.2 (15N average = 3.7) and showed no obvious trend. The relative contribution of marine and terrestrial detrital material deposited within these sediments was inferred by comparing isotopic compositions to C/N values. The significant contribution of inorganic nitrogen (Nbound average 75%), as estimated from total organic carbon/total nitrogen plots, likely lowered the observed C/N values.

We conducted temperature and pore pressure measurements using the Davis-Villinger Temperature-Pressure Probe and the temperature/dual pressure probe penetrometers during Integrated Ocean Drilling Program Expedition 308. In Ursa Basin, 18 measurements were used to determine that the geothermal gradient at Site U1324 is bilinear. The temperature gradient is 18.6Â°C/km in lithostratigraphic Unit I and 16.7Â°C/km in Unit II. Based on nine measurements at Site U1322, the geothermal gradient is 21.9Â°C/km. In Brazos-Trinity Basin IV, the geothermal gradient at Site U1320 is 23.1Â°C/km. In Ursa Basin, significant overpressures (overpressure ratio = ~0.7) are observed in the sediments above ~200 meters below seafloor (mbsf) at Sites U1322 and U1324. At Site U1324, pore pressure decreases with increasing depth between 200 and 300 mbsf. Below 300 mbsf and within lithostratigraphic Unit II, overpressure is approximately constant (~1 MPa). Unit II is composed of silty claystone interbedded with beds of silt and very fine sand. In Brazos-Trinity Basin IV, only two penetrometer deployments were made and the data are inconclusive.

We conducted temperature and pore pressure measurements using the Davis-Villinger Temperature-Pressure Probe and the temperature/dual pressure probe penetrometers during Integrated Ocean Drilling Program Expedition 308. In Ursa Basin, 18 measurements were used to determine that the geothermal gradient at Site U1324 is bilinear. The temperature gradient is 18.6Â°C/km in lithostratigraphic Unit I and 16.7Â°C/km in Unit II. Based on nine measurements at Site U1322, the geothermal gradient is 21.9Â°C/km. In Brazos-Trinity Basin IV, the geothermal gradient at Site U1320 is 23.1Â°C/km. In Ursa Basin, significant overpressures (overpressure ratio = ~0.7) are observed in the sediments above ~200 meters below seafloor (mbsf) at Sites U1322 and U1324. At Site U1324, pore pressure decreases with increasing depth between 200 and 300 mbsf. Below 300 mbsf and within lithostratigraphic Unit II, overpressure is approximately constant (~1 MPa). Unit II is composed of silty claystone interbedded with beds of silt and very fine sand. In Brazos-Trinity Basin IV, only two penetrometer deployments were made and the data are inconclusive.

We describe the response of a compressible submarine hydrologic monitoring instrument to formation pressure changes in low-diffusivity rock. The measured pressure depends on the frequency of the pressure signal, the hydraulic diffusivity, and the wellbore storage. The Nankai advanced circulation obviation retrofit kits (ACORKs) (offshore Japan) record tide-induced formation pressure changes with small amplitudes (<10% of seafloor amplitudes) and large phase shifts (>25Â°). The pressure measurements occur in thick, homogeneous, compressible, low-permeability sediment, where in situ tidal pressure responses should approximate the seafloor tidal signal. A wellbore storage of 2 Ã 10&#8722;8 m3 Pa&#8722;1 can explain many of the observed tidal responses, given the hydraulic diffusivities of the monitored intervals. A reduced permeability around the wellbore of 1000-fold and a wellbore storage of 10&#8722;11 m3 Pa&#8722;1 can also reconcile the data. Our analysis suggests that ACORK screens in the Lower Shikoku Basin facies have a critical frequency on the order of 5 Ã 10&#8722;8 Hz (equivalent to a period of 250 days); higher-frequency formation pressure signals will be distorted in the pressure record. Within the Lower Shikoku Basin facies the time for this monitoring system to record 90% of an instantaneous pressure change is on the order of 10 d. We suggest that the ACORK instrument compliance contributes to, but does not fully explain, the small tidal amplitudes and large phase shifts recorded at the least permeable monitoring intervals.

We developed a multicomponent, multiphase, fluid and heat flow model to describe hydrate formation in marine sediments; the one- and two-dimensional model accounts for the dynamic effects of hydrate formation on salinity, temperature, pressure, and hydraulic properties. Free gas supplied from depth forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation: Salinity and hydrate concentration increase upward from the base of the regional hydrate stability zone (RHSZ) to the seafloor, and the base of the hydrate stability zone has significant topography. In fine-grained sediments, hydrate formation leads to rapid permeability reduction and capillary sealing to free gas. This traps gas and causes gas pressure to build up until it exceeds the overburden stress and drives gas through the RHSZ. Gas chimneys couple the free gas zone to the seafloor through high-salinity conduits that are maintained at the three-phase boundary by gas flow. As a result, significant amounts of gaseous methane can bypass the RHSZ, which implies a significantly smaller hydrate reservoir than previously envisioned. Hydrate within gas chimneys lies at the three-phase boundary, and thus small increases in temperature or decreases in pressure can immediately transport methane into the ocean. This type of hydrate deposit may be the most economical for producing energy because it has very high methane concentrations (S h > 70%), located near the seafloor, which lie on the three-phase boundary.

Two borehole penetrometers, Fugro-McClelland's Piezoprobe and the Ocean Drilling Program's (ODP) DVTPP, were deployed 50 m below seafloor at Site 1244 on ODP Leg 204 to measure formation pressure at southern Hydrate Ridge, offshore Oregon. Pore pressure is interpreted to be hydrostatic and the sediment's coefficient of consolidation is interpreted to lie between 6.92 and 7.8 ÃÂÃÂÃÂÃÂÃÂÃÂÃÂ¢ÃÂÃÂÃÂÃÂÃÂÃÂ¢ÃÂÃÂ¢ÃÂ¢ÃÂÃÂÃÂÃÂ¬ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂÃÂ 10&#8722;7 m2/s, which is in approximate agreement with laboratory measurements. The Piezoprobe pressure reaches 90% of dissipation 14 times sooner than the DVTPP. The observed and modeled pore pressure responses illustrate how penetrometer geometry impacts our ability to interpret in situ properties and demonstrate under what conditions these tools can be effectively used. Because of its narrow tip, the Piezoprobe disturbs a narrower zone than the DVTPP does. This generates a narrower zone of pressure increase around the piezoprobe, which dissipates much faster than the DVTPP. As consolidation proceeds, pressure dissipation of the Piezoprobe is retarded and forms a ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ¢ÃÂÃÂÃÂÃÂ¢ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂ ÃÂ¡ÃÂÃÂÃÂÃÂ¬ÃÂÃÂÃÂ¢ÃÂÃÂ¦ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂ ÃÂbench,ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ¢ÃÂÃÂÃÂÃÂ¢ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂ ÃÂ¡ÃÂÃÂÃÂÃÂ¬ÃÂÃÂÃÂ¢ÃÂÃÂÃÂÃÂÃÂÃÂ or flat spot, on the dissipation curve. Owing to its distinct two-radius geometry, it is possible to apply a consistent method to estimate in situ pressure from partial dissipation record based on the position of the ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ¢ÃÂÃÂÃÂÃÂ¢ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂ ÃÂ¡ÃÂÃÂÃÂÃÂ¬ÃÂÃÂÃÂ¢ÃÂÃÂ¦ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂ ÃÂbench.ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ¢ÃÂÃÂÃÂÃÂ¢ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂ ÃÂ¡ÃÂÃÂÃÂÃÂ¬ÃÂÃÂÃÂ¢ÃÂÃÂÃÂÃÂÃÂÃÂ

The interplay between sedimentation and erosion during the late Pleistocene in the Mars-Ursa region, northern Gulf of Mexico, resulted in a complex compartmentalized reservoir. Rapid deposition, directly downdip of the Mississippi River beginning about 70 k.y., quickly filled antecedent topography in the Mars-Ursa region with a thick accumulation of sand and mud called the blue unit. This permeable reservoir was rapidly and asymmetrically buried by thick, mud-rich levees of two channel-levee systems. Both systems plunged from north to south with a steeper gradient than the underlying blue unit. Rotated channel-margin slides present in both channel-levee systems rotated low-permeability, mud-rich levee deposits beneath the sand-rich channel fill. As a result of the channel-levee systems, the east-west hydraulic connectivity of the blue unit decreases progressively from north to south until its eastern and western halves become completely separated.

Log and core data document gas saturations as high as 90% in a coarse-grained turbidite sequence beneath the gas hydrate stability zone (GHSZ) at south Hydrate Ridge, in the Cascadia accretionary complex. The geometry of this gas-saturated bed is defined by a strong, negative-polarity reflection in 3D seismic data. Because of the gas buoyancy, gas pressure equals or exceeds the overburden stress immediately beneath the GHSZ at the summit. We conclude that gas is focused into the coarse-grained sequence from a large volume of the accretionary complex and is trapped until high gas pressure forces the gas to migrate through the GHSZ to seafloor vents. This focused flow provides methane to the GHSZ in excess of its proportion in gas hydrate, thus providing a mechanism to explain the observed coexistence of massive gas hydrate, saline pore water and free gas near the summit.

2-D seismic, wireline log, and core data at ODP Leg 174A Sites 1071 and 1072 on the outer continental shelf of New Jersey reveal two major depositional sequences of late MioceneâPliocene and Pleistocene age. The late MioceneâPliocene sequence is a thick (not, vert, similar100 m) deepening-upward succession landward of the clinoform rollover and a shoaling-upward succession seaward of the clinoform rollover. The Pleistocene sequence deepens abruptly near its base, shoals upward, and then deepens again before it is truncated by its overlying unconformity. There is no onlap onto clinoforms (no lowstand wedge) in either sequence. Sequence stratigraphic analysis and a geometric depositional model are used to interpret that the unusually thick transgressive component of the late MioceneâPliocene sequence was formed by high-frequency eustatic cycles (1â2 m.y.) superimposed on a longer-term eustatic rise (not, vert, similar5 m.y.). This conclusion is supported by independent evidence of eustasy. The sequences of this study are correlated to sequences in the North Atlantic coastal plain and in the Great Bahama Bank. These sequences have very different architectures than underlying middle Miocene sequences, which contain thick lowstand wedge deposits, and are interpreted to have formed by high-frequency eustatic cycles superimposed on longer-term eustatic fall.